Add Mixotoxodons (Mixotoxodon cf larensis) to the list of Mammals that Occupied Southeastern North America during the Pleistocene

Toxodons and the closely related mixotoxodons originally evolved on the South American continent.  They are classified as notoungulates– primitive hooved animals that resembled the ancient ancestors of cattle, antelope, deer, horses, hogs, etc.  Artistic representations of notonungulates are reminiscent of beasts known from the Eocene and Oligocene.  Mixotoxodons weighed a little over a ton, ate plant material, and looked kind of like a rhino or hippo.

Mixotoxodon.jpg

Mixotoxodons were notoungulates, a group of primitive ungulates that evolved in isolation from all other hooved animals when South America was an island continent.

The notoungulates were thought to be restricted to tropical climates.  Until 2004 fossil remains were not known north of Guatemala. But that year, mixotoxodon fossils were excavated from 2 sites in Mexico–Hihiuctilan, Michoacán and La Estribera, Veracruz.  This was a range extension of 900 miles north but still well within the tropics.  During 2012 a mixotoxodon tooth (un upper 3rd left molar) was discovered along Cypress Creek, Harris County, Texas–a further range extension of 800 miles north.

State of Michoacán within Mexico

Michoacán Province in Mexico.  Mixotoxodon fossils were found here for the first time about a decade ago.  At the time they were the northernmost known fossils of this species.

Map of Texas highlighting Harris County

Harris County, Texas.  A fossil tooth of a mixotoxodon was found here just a few years ago.  Now this site is the northernmost known locality where toxodons once lived.

Harris County, Texas borders the modern day temperate and subtropical zone.  The mixotoxodon tooth couldn’t be directly dated but 2 pieces of fossil oak wood located 10 meters upstream from the specimen were dated and yielded ages of 17,080 years BP and 23,780 years BP.  The tooth is believed to be from the same geological strata as the fossil wood.  If the tooth does belong within this time span, than mixotoxodons occurred in southeast Texas during the Last Glacial Maximum, the coldest phase of the last Ice Age.  However, climate in this region then was likely similar to that of the present day, though summers were probably a bit cooler and winters a little warmer.  The modern day oceanic gulf stream that moderates present day climates in eastern North America didn’t exist during the coldest phases of the Ice Age.  Instead, tropically heated water remained near the equator and may have upwelled into the Gulf of Mexico at times.  When this warm water circulated just a little north, it may have reduced the intensity and frequency of frosts near the southern Atlantic coast and around the coast along the Gulf of Mexico.  The extent of warm water upwelling in this region likely fluctuated cyclically, resulting in decades of warm winters with few local frosts alternating with decades when frosts occurred more often because the warm waters pooled near the equator and away from nearby coasts.

I see no ecological reason why mixotoxodons couldn’t have occurred further east in North America.  Coastal savannah interspersed with open woodlands along waterways stretched from Mexico to Florida.  The Harris County mixotoxodon tooth was found right in the middle of where this type of environment existed.  Why isn’t there more evidence of mixotoxodons in North America, especially from the abundant fossil sites in Florida?  The fossil record is not a complete accounting of every organism that ever lived in a location, perhaps explaining the absence of this species.  Collared peccaries and giant short-faced bears were unknown from Florida’s fossil record until just a few years ago, so maybe some day someone will find a mixotoxodon fossil elsewhere in the south.  Alternatively, the Harris County specimen may represent a very temporary range extension of a cold sensitive species that occurred during a few decades or centuries when an upwelling of tropically warmed water resulted in a brief cycle of locally frost free winters.

Below is a list of currently known land mammals, weighing over 100 pounds, that lived in southeastern North America during the late Pleistocene.

1. Mixotoxodon (Mixotoxodon cf larensis)–from just 1 tooth certain species identification is not possible

2. Columbian mammoth (Mammuthus colombi)

3. woolly mammoth (Mammuthus primigenius)–not known farther south than Virginia

4. mastodon (Mammut americanum)

5. gompothere (Cuverionius tropicalis)

6. upland bison (Bison antiquus)

7. long-horned bison (Bison latifrons)

8. helmeted musk ox (Bootherium bombifrons)

9. collared peccary (Pecari tajadu)

10. long-nosed peccary (Mylohyus nasatus)

11. flat-headed peccary (Platygonus compressus)

13. stout-legged llama (Paleolama mirifica)

14. long -necked llama (Hemiauchenia macrophela)

15. stag-moose (Cervalces scotti)–from a site in South Carolina and a site in Mississippi

16. elk (Cervus canadensis)

17. caribou (Rangifer tarandus)

18. white tailed deer (Odocoilus virginiana)

19. horse (Equus ?)

20. half-ass (Equus?)

21. Vero tapir (Tapirus veroensis)

22. giant ground sloth (Eremotherium laurillardi)

23. Harlan’s ground sloth (Paramylodon harlani)

24. Jefferson’s ground sloth (Megalonyx jeffersonii)

25. pampathere (Holmesima septentrionalis)

26. glyptodont (Glyptodont floridanum)

27. giant beaver (Casteroides dilophidus)

28. capybara (Hydrochoreus)

29. another species of capybara (Neochoreus)

30. giant short-faced bear (Arctodus simus)

31. spectacled bear (Tremarctos floridanus)

32. black bear (Ursus americanus)

33. dire wolf (Canis dirus)

34. saber-tooth (Smilodon fatalis)

35. scimitar-tooth (Dinobastis serum)

36. giant lion (Panthera atrox)

37. jaguar (Panthera onca)

38. cougar (Puma concolor)

39. human (Homo sapiens)

References:

Lundelius, Ernest; et. al

“The First Occurrence of a Toxodont (Mammalia; Notoungulata) in the U.S.”

Journal of Vertebrate Paleontology 33 (1) Jan 2013

De la Rosa, Ruben; Jose Guzman-Gutierrez and Carlos Hurtado Menoza

“A New Occurrence of Toxodonts in the Pleistocene of Mexico”

Current Research in the Pleistocene 2011

 

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